1. Field of the Invention
The present invention relates to photographic emulsions by which radiation images (including X rays, electron rays, ultraviolet rays, visible rays and infrared rays) can be recorded. In greater detail, the present invention relates to silver halide photographic emulsions having improved photosensitive properties (sensitivity and sensitive range) to radiation.
2. Description of the Prior Art
The prior art concerning sensitization of silver halide photosensitive materials can be divided into chemical sensitization and spectral sensitization. In chemical sensitization, various sulfur compounds, for example, compounds described in U.S. Pat. Nos. 1,574,944, 1,623,499 and 2,410,689; various gold salts (including complex salts), for example, the compounds described in U.S. Pat. Nos. 2,597,856 and 2,597,915 such as potassium chloroaurate and auric chloride; and reduction compounds, for example, the compounds described in U.S. Pat. No. 2,487,850 such as stannous chloride are used for silver halide emulsions. With chemical sensitization, it is possible to increase the sensitivity of the silver halide in the inherent sensitivity wavelength range of silver halide.
In spectral sensitization, it is possible to provide photosensitive properties in a wavelength range which is on a longer wavelength side of the inherent sensitivity wavelength range of silver halide by adding polymethine dyes (e.g., aninidium ion type cyanine dyes or merocyanine dyes) alone or together with other dyes to silver halide emulsions. Spectral sensitizers suitable for such purpose have been described in detail in F. M. Hamer The Cyanine Dyes and Related Compounds, Interscience Publishers (1964) and C. E. K. Mees & T. H. James The Theory of the Photographic Process 3rd Ed., The Macmillan Company, (1966).
In both chemical sensitization and spectral sensitization, the sensitizers are used in a small amount, i.e., as little as 10.sup.-.sup.6 - 10.sup.-.sup.4 mol of sensitizer per mol of silver halide.
If used in a large amount, undesirable phenomena such as a generation of fog or a decrease of sensitization efficiency occur.